Solar energy is an abundant renewable energy source. It has been estimated that the sun deposits more than 12,000 terrawatts (TW) of energy on Earth, which is large compared to the 13 TW of total current power consumption worldwide. Thus, converting even 0.1% of the sunlight into useful electricity could eventually meet the total current power consumption worldwide.
There are various ways to capture, store and convert solar radiation in useful forms of energy. Solar cells are one way to directly convert sunlight into electricity. Various materials and technologies have been developed that involve single crystal, polycrystalline, amorphous and nanostructured inorganic and organic materials for use in solar cells. More recently, multi-junction cells, quantum dots and dye sensitized photovoltaics have received attention within the solar cell community. However, these solar cells are generally considered to have relatively low conversion efficiencies. Solar cells made from Gallium Arsenide (GaAs) materials show efficiencies about as high as 25-30%, although some laboratory devices have reported efficiencies of up to 40% or more, as in the Reference Report of the Basic Energy Sciences Workshop on Solar Energy Utilization, Basic Research Needs for Solar Energy Utilization, Sponsored by the Office of Science, U.S. Department of Energy, April 18-21, Bethesda, Md.
Simply stated, a solar cell includes an “antenna” which captures (or absorbs) the solar radiation and a rectifying element which converts the captured wave (photon) into a direct current (DC) output.
For semiconductor based photovoltaic solar cells, the DC power output is highly material dependent (e.g., bandgap and resistivity) and there is a maximum theoretical efficiency (of about 41%) to capture and convert solar energy. This theoretical limit of the solar cell results because (i) the incident light with energy less than the bandgap can not be absorbed; and (ii) for all the incident light with energy equal to or larger than the bandgap, only the energy equal to the bandgap can be efficiently converted into a useful DC current. While it may be possible to increase the maximum obtainable efficiency by using multi-junction solar cells, this requires increased complexity and cost.
Thus, there is a need for an improved solar-to-electric conversion device having higher conversion efficiencies that are commercially viable. In particular, there is a need for solar energy conversion devices in which the maximum theoretical collection efficiency does not depend on the bandgap energy of the collector in the solar cell or equivalently of the incident photon energy or wavelength.
A rectenna (rectifying antenna) is a device which includes a receiving antenna and a rectifying diode. Rectennas have been investigated in the microwave region for power transmission and detection. Applications have included long distance power beaming, signal detection and wireless control systems. Within the microwave region, it is believed that the greatest conversion efficiency may have been achieved by a rectenna element in 1977 by Brown, Raytheon Company. Using a GaAs—Pt Schottky barrier diode, a 90.6% conversion efficiency was recorded with an input microwave-power level of 8 Watts. However, conversion efficiencies in the range of about 80% are believed to be more representative.
It has been suggested that the rectenna concept is arbitrarily scaleable. That is, the optical rectenna is a direct extension to shorter wavelengths. Some recent work with such rectennas in the area has been performed by ITN Energy Systems under DOE and DARPA sponsorship. ITN Energy Systems assertedly demonstrated that such micro-scale and nano-scale rectenna devices can convert simulated solar radiation to DC electric power. ITN Energy Systems has asserted that the rectenna devices using improved metal-insulator-metal (MIM) diodes may be expected “to yield much higher efficiencies (>85%).” Even though there are no confirmed experimental results with these efficiencies, by extrapolation from the microwave region, ITN estimates that for devices operating in the solar spectrum region, there exists the potential to convert over 85% of the sun's energy to useable power compared to the 10-30% that is presently achievable with standard semiconductor based photovoltaics, such as GaAs.
It is believed, however, that useful power conversion in the optical frequency range may be limited by the low frequency response of the planar diodes employed in the foregoing studies (in particular, the diodes were 100 nanometer square in the ITN device). Prior efforts to develop solar cells using standard two-element rectennas are believed to have had limited success due to the frequency limited response of the planar MIM diodes.
It is believed that the problem of frequency response of the planar MIM diodes (which are limited by parasitic capacitance effects) may be overcome by using point-contact nanowires or metallic carbon nanotubes (mCNTs), each acting as an antenna and a rectifier. Point-contact devices (for example, whisker diodes) have been used to measure absolute laser frequencies up to the green part of the visible spectrum, and have demonstrated a response time on the order of femtoseconds, which is orders of magnitude faster than conventional MIM diodes. A whisker is a metal wire with a sharp edge that is sub-micron in size.
Hung Quang Nguyen measured the frequency response of a laser irradiated STM junction by changing the spacing of the junction. The results show that beyond about 1-3 nanometers, the emitted current drops significantly, which indicates that fewer electrons are able to tunnel through the barrier due to the change in polarity of the oscillating electric field, as discussed by Hung Quang Nguyen in Experimental and Theoretical Studies of Tunneling Phenomena Using Scanning Tunneling Microscopy and Spectroscopy, Ph. D. Thesis, available from University Microfilms International, now called Bell and Howell Information and Learning (1989).
A three-dimensional quantum mechanical study of the properties of point-contact diode structures (for both mCNTs and nanowires) as a rectenna for application as a solar cell is discussed in the article by A. Mayer, M. S. Chung, B. L. Weiss, N. M. Miskovsky, and Paul H. Cutler, Three-Dimensional Analysis of the Geometrical Rectifying Properties of Metal-Vacuum-Metal Junctions and Extension for Energy Conversion, Phys. Rev. B. 77, 085411 (Feb. 8, 2008), which is incorporated by reference.
In the articles by Peter Burke, Shendong Li, and Zhen Yu, Quantitative Theory of Nanowire and Nanotube Antenna Performance, 5 IEEE Trans. on Nanotech. 314 (2006) and P. J. Burke, A Luttinger Liquid Theory as a Model of the Gigahertz Electrical Properties of Carbon Nanotubes, 1 IEEE Trans. on Nanotech. 129 (2002), the antenna properties of mCNTs were analyzed by modeling the electron gas as a Luttinger liquid (i.e., 1-D interacting electron gas). The nanowires/mCNTs were treated as 1-D conductors in antenna and transmission line configurations by classical antenna theory. The addition of two electrical and magnetic contributions were included in treating the nanowire/mCNT circuit elements. These are a quantum capacitance and a kinetic inductance, which is analogous to the classical magnetic inductance. In the Burke article, the multi-walled CNT was treated as an antenna (within classical antenna theory), but included these additional contributions as lumped circuit elements.
Mayer's quantum mechanical calculations include Burke's quantum mechanical corrections. In particular, the Mayer article discusses the determination of the static and dynamic tunneling barriers, for static and induced AC fields due to incident radiation for the 440 nanometer green line in the optical region. These calculations were done for similar and dissimilar combinations of metallic tips and bases (i.e., anode or planar electrodes). Temperature asymmetries between the tips and the bases were also included, and results were obtained as a function of tip radius and tip-anode separation. Calculated rectified currents were obtained leading to a DC voltage across an external load.
Properties of the currents and voltages developed in a whisker-type-nanowire antenna have been discussed by T. E. Sullivan, Paul H. Cutler, and A. A. Lucas in The Use of Antenna Theory to Calculate the Electric Fields in a Thermal Field Emission Metal Whisker Diode, Surface Sci. 62, 455 (1977) and by T. E. Sullivan in Thermal and Field Emission Effects of Laser Radiation on Meal Whisker Diodes: Application to Infrared Detection Devices, Ph.D. Thesis, University Microfilms International (now called Bell and Howell Information and Learning) (1977), which are incorporated by reference. The latter Sullivan article discusses the calculation of fields and voltages induced on different metal whisker tips by incident laser radiation, and the use of linear antenna theory to describe the receiving properties of the whiskers.
Planar and cylindrical antennas are known in the field of antenna theory, as discussed by Keith R. Carver and James W. Mink in Microstrip Antenna Technology, IEEE Trans. Antennas and Propogation, AP-29, No. 1, 2 (1981).
The actual pointed geometry of the tip is taken into account using Schelkunoff's theory of the bi-conical antenna, and is discussed, for example, in Advanced antenna theory, New York: John Wiley & Sons, 1952. The strength of the electric fields are found to be comparable to the strength necessary for field emission (F≧107V/cm). As discussed by T. E. Sullivan in Thermal and Field Emission Effects of Laser Radiation on Meal Whisker Diodes: Application to Infrared Detection Devices, Ph.D. Thesis, University Microfilms International (now called Bell and Howell Information and Learning) (1977), the highest fields are established on gold tips. This is consistent with the experiments of Green, who found that the best response occurs with gold-gold contacts, as discussed by S. J. Green in Point Contact MOM Tunneling Detector Analysis, 142 Appl. Phys. 1166 (1971). This theory predicts that the detection sensitivity is determined by the Schelkunoff's radiation reactance.
The article by A. Mayer, M. S. Chung, B. L. Weiss, N. M. Miskovsky, and Paul H. Cutler, Three-Dimensional Analysis of the Geometrical Rectifying Properties of Metal-Vacuum-Metal Junctions and Extension for Energy Conversion, Phys. Rev. B. 77, 085411 (Feb. 8, 2008), which is incorporated by reference, discusses performing a three-dimensional analysis of the geometrical rectifying properties of metal-vacuum-metal (MVM) junctions (and implicitly MIM junctions), in which the lower metal supports a hemi-spherical protrusion that simulates a tip. Because of this geometrical asymmetry, the system assertedly exhibits rectification properties that were examined by using a transfer-matrix methodology, which took into account three-dimensional aspects of the potential barrier. These results assertedly demonstrate that junctions presenting either a geometrical, a material or a thermal asymmetry exhibit rectification properties, which enable the production of DC currents from an oscillating field.
FIGS. 7a and 7b show the trends in measured DC output determined experimentally by Hung Quang Nguyen in Experimental and Theoretical Studies of Tunneling Phenomena Using Scanning Tunneling Microscopy and Spectroscopy, Ph. D. Thesis, available from University Microfilms International (now called Bell and Howell Information and Learning) (1989). In FIG. 7a, the DC output over a Scanning Tunneling Microscope (STM) junction was measured as the spacing between the tip and the anode was held constant and the diode was irradiated with lasers of various frequencies. In FIG. 7b, the frequency of the incident radiation was held constant as the tip-anode spacing was varied. The graph of FIG. 7a shows that the measured DC output decays as the frequency v of the incident radiation rises and the tip-anode spacing S is held constant. The graph of FIG. 7b shows that the measured DC output decays as the tip-anode spacing S rises and the frequency v of the incident radiation is held constant. FIGS. 7a and 7b show that electrons in the tip (charged by the incident radiation) cannot penetrate the barrier of the STM junction if the tunneling junction is too long or if the electric field oscillates too quickly.